The present exemplary embodiment relates to lanthanide ion complexes. It finds particular application in conjunction with complexes that absorb or fluoresce in the visible or near-infrared (NIR) region of the electromagnetic spectrum, a process for preparing such complexes, and their NIR emission properties that render the complexes useful in imaging applications such as methods of imaging or therapy using such complexes. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Imaging techniques are used for a variety of applications, including drug discovery and preclinical testing, studies of disease, treatment and medical diagnosis. Molecular imaging is a rapidly emerging field, as it provides noninvasive visual quantitative representations of fundamental biological processes (T. F. Massoud, S. Gambhir, “Integrating noninvasive molecular imaging into molecular medicine: an evolving paradigm,” Trends in Molecular Medicine 2007, 13, 183-191). Molecular imaging differs from conventional diagnostic imaging in that it uses probes known as biomarkers, which interact chemically with their surroundings and give signals according to molecular changes/response occurring within the area of interest. This ability to image fine molecular changes can directly or indirectly reflect specific cellular and molecular events that can reveal pathways and mechanisms responsible for disease (R. Weissleder, V. Ntziachristos, “Shedding light onto live molecular targets,” Nature Medicine 2003, 9, 123-128). It is assumed that molecular probes/markers may serve as early indicators of a disease process, long before a pathomorphological transformation of tissue occurs. Applications include visualization of biodistribution of drugs/ligands, of cell migration for evaluating cell therapies, and the expression of drug targets (receptors, enzymes).
Recently, there has been an increasing interest in identifying luminescent chemosensors for medical diagnostic applications (See, for example, J. Zhang, R. E. Campbell, A. Y. Ting; R. Y. Tsien, “Creating new fluorescent probes for cell biology,” Nature Reviews. Molecular Cell Biology 2002, 3, 906-918). Some of the reasons for the interest are that luminescence-based imaging is non-invasive, involves non-ionizing radiation, and can provide high sensitivity, thus combining some of the best qualities of PET (positron emission tomography), SPECT, ultrasound, and MRI. Optical imaging uses the fluorescence as optical contrast. Like ultrasound, optical imaging does not have strong safety concerns in comparison with the other medical imaging modalities, which is a valuable attribute (E. M. Sevick-Muraca, J. C. Rasmussen, “Molecular Imaging with Optics: Primer and Case for Near-Infrared Fluorescence in Personalized Medicine,” Journal of Biomedical Optics 2008, 13, 041303-1-041303/16).
There has been some progress in the design and synthesis of fluorescent probes, enabling detection and imaging of molecular events in various disease conditions such as cancer and vascular pathophysiology. (See, for example, S. Achilefu, “Lighting up Tumors with Receptor-Specific Optical Molecular Probes,” Technology in Cancer Research & Treatment 2004, 3, 393-409; and J. Klohs, et al., “Near-infrared fluorescent probes for imaging vascular pathophysiology,” Basic Research in Cardiology 2008, 103, 144-151).
Fluorescent molecules that absorb and emit light in the near-infrared (NIR) region are of particular interest for potential in vivo imaging applications. For biological tissues, the spectral range of interest is approximately 850-1100 nm, where the background noise arising from the fluorescence of the biological material itself (cellular autofluorescence) noise is minimal. During fluorescence microscopy, the fluorophores are subject to photo-irradiation and detectability is limited by cellular autofluorescence and auto-absorption. One approach to overcoming the autofluorescence problem is to develop fluorescent probes that display long emission wavelengths, long decay times, and high quantum yield and high fluorescence brightness (see, for example, Z. Gryczynski, et al., Long-wavelength long-lifetime luminophores for cellular and tissue imaging. In Proceedings of SPIE, Volume 5323: Multiphoton Microscopy in the Biomedical Sciences; P. T. C. So, ed. 2004; pp 88-98).
The use of lanthanide chelates as luminescent labels has been increasingly recognized as a technique for detecting biomolecules with high sensitivity. One feature of lanthanide chelate luminescence is that the excited state lifetime is unusually long (often over 1 millisecond) in comparison with the lifetime of organic fluorescent compounds. Therefore, time-resolved fluorometric measurement of lanthanide chelate compounds eliminates the undesired background fluorescence, which decays within several nanoseconds. Other attractive features of lanthanide chelates are their emission in the NIR region, narrow emission bands which originate from the f-f transition of the lanthanide atom, and high detection sensitivity.
The lanthanide elements (abbreviated herein as Ln) are considered to be the sequence of 15 elements with atomic numbers from 57 (lanthanum) to 71 (lutetium). All lanthanide elements are f-block elements, corresponding to the gradual filling of the 4f electron shell. The characteristic f→f transitions are quite narrow, and substantially unaffected by the chemical environment of the ion. These transitions are easily recognizable, making lanthanide ions candidates for optical probes. Most of the lanthanide cations are luminescent, either fluorescent (e.g., Pr3+, Nd3+, Ho3+, Er3+, and Yb3+) or phosphorescent (e.g. orange Sm3+, red Eu3+, green Tb3+, and blue Tm3+). Their emission colors cover the entire spectrum from UV-visible to near-infrared (NIR) region (300-2200 nm). The f-f transitions, however, have low absorption coefficients (smaller than 10 M−1 cm−1), since the electric dipole selection rules forbid such a transition. This hampers the use of lanthanide ions in imaging. In a lanthanide ion complex, however, interaction between the 4f orbitals and the surrounding ligand orbitals provides a mechanism for the energy transfer from the binding ligand (at the excited states) to the lanthanide ions. This indirect excitation process, termed sensitization or antenna effect, can excite the lanthanide ions, which then give NIR emission. To be used in the sensitization, the ligands need to provide efficient energy transfer to the Ln(III) ions.
Among the promising ligands used for NIR sensitization of Ln(III) are 8-hydroxyquinoline derivatives (See, for example, U.S. Pat. No. 6,277,841; and Inorg. Chem. 45, 732-743 (2006), Chemistry—A European Journal 13, 936-944, (2007)), substituted 2-quinolinols (see, for example, U.S. Pat. No. 7,297,690), porphyrin derivatives (Coordination Chemistry Reviews 251, 2386-2399 (2007), and tropolonate ligands (see, for example, J. Zhang, P. D. Badger, S. J. Ceib; S. Petoud, Angew. Chem, Int. Ed. 2005, 44, 2508-2512).
However, NIR signals generated with such ligands can be weak and can be masked by autofluorescence signals in imaging.
There remains a need for lanthanide complexes with long decay times which are readily detectable from their luminescence properties.